An optical network involves transmission of a wavelength-division multiplexed (WDM) light consisting of a plurality of wavelengths corresponding to a plurality of optical channels or light paths arranged at 100-GHz spacing, for example, over a predetermined optical bandwidth. At each node of the above-indicated optical network, optical signals of the wavelengths of the optical channels included in the wavelength-division multiplexed light beam and transmitted through optical fibers or the like are separated and multiplexed, and switched or routed into optical fibers for further transmission in a predetermined direction. Recently, the number of optical channels processed by an optical switching device at the above-indicated node has been increasing with an increase of a required capacity of a communication system, so that the communication system tends to utilize the wavelength-division multiplexed light beam consisting of a plurality of wavebands each including a plurality of optical channels.
In view of the above, there has been considered an optical network wherein each node is configured such that not only the optical channels of the optical signals are separated and multiplexed, and routed into the optical fibers for transmission in the predetermined direction, but also the wavebands of the optical signals are demultiplexed and multiplexed, and routed into the optical fibers for transmission in the predetermined direction. For demultiplexing and multiplexing the plurality of wavebands included in the wavelength-division multiplexed light beam, it has been a conventional practice to use a multi-layered thin-film interference filter type multiplexer/demultiplexer, an acoust-optic tunable filter (AOTF) type multiplexer/demultiplexer, etc., which are disclosed in non-patent Document 1, non-patent Document 2, non-patent Document 3, non-patent Document 4 and non-patent Document 5.    Non-Patent Document 1:    “Ultra-low stress coating process: an enabling technology for extreme performance thin film interference filters” OFC 2002 Postdeadline Papers, PA8-1    Non-Patent Document 2:    “Recent Progress on AO Devices” Thesis Journal of Society of Electronic Information and Communication, VOL. J86-C, pp. 298-308, Published in December, 2003    Non-Patent Document 3:    “Full-mesh Network Utilizing Cyclic Characteristic of Wavelength Response” NTT R & D VOL. 49 No. 6, pp 298-308, Published in June, 2000    Non-patent Document 4:    “Integrated Band Demultiplexer Using Waveguide Grating Routers” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 8, AUGUST 2003    Non-Patent Document 5:    “Flexible Waveband Optical Networking Without Guard Bands Using Novel 8-Skip-0 Banding Filters” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 3, MARCH 2005    Patent Document 1: JP-11-30730A    Patent Document 2: JP-2006-11345A